I'm not particularly certain myself. It could be as Derp Mason suggested, because a majority of people like Minecraft and so maps associated to it will attract more players. However, I feel whilst this is a reason, that the main reason would be that Minecraft maps do not require pre-purchased third-party textures. As the other servers all have textures from games such as CSS; if I'm correct.
Because Minecraft maps don't require CSS textures, so to play on Vanilla you don't need to have CSS so... yeah.